The invention relates to an erasable optical recording element having a substrate and a recording layer provided thereon in which information bits in the form of amorphous areas in crystalline surroundings are formed by exposure to modulated laser light, which information bits can be read optically and can be reconverted into the crystalline phase by exposure to laser light and are thus erased.
An erasable optical recording element of this type the operation of which is based on crystalline-amorphous transitions (phase change) is known, for example, from European patent application No. 0 135 370 and from the co-pending Netherlands patent application No. 8403817 in the name of the Applicants. In the known recording elements a Te-Se alloy which may be include other elements is used as the recording material. The known recording material such as a Te-Se-Sb alloy has as an disadvantage that the erasing time, i.e. the exposure time which is necessary to revert the amorphous information area to the original crystalline phase, is long. For example, the erasing time of a Te-Se-Sb alloy is, for example, 50 .mu.s.
During recording information, reading the information and erasing the information, the recording element is rotated. The linear speed of the elements is, for example, from 1 m/s to 15 m/s or more. For example, upon recording audio information according to the EFM (eight out of fourteen) modulation system a linear disc (element) speed is used of 1.4 m/s. For video information a linear speed of 10 to 15 m/s is used.
It is highly desirable to perform the erasing process at the same linear disc speed as the recording process, i.e. erasing in real time. It is furthermore desirable to be able to erase the information during one rotation of the element. For the practical application this provides very interesting possibilities to record new information directly over the existing information, the new information being recorded immediately after the existing information has been erased at the same disc speed and in the same run of the element. Such a direct re-recording (overwrite) process cannot be realized in magneto-optical recording processes in which at least one complete revolution of the recording element lies between erasing and re-recording.
In phase change recording with the above-mentioned erasing time of 50 .mu.s, an erasing process in real time within one revolution of the element is also not practically possible. Even at a very low linear disc speed of 1.4 m/s a slot-shaped erasing spot having a length of 70 .mu.m must be used. The erasing spot is the light spot of the laser light beam on the recording layer used in the erasing process. There are considerable practical objections to the use of an erasing spot of 70 .mu.m due to the required high laser power as well as the positioning of the spot on the track. At higher disc speeds, even an erasing spot having a length in the order of magnitude of mm is necessary, which is practically not possible.